1. Field of The Invention
The present invention relates generally to an image fixing device for an image forming apparatus, such as an electrophotographic copying machine or a laser-beam printer. More particularly, the invention relates to an image fixing device for fixing a toner image to a toner sheet by heat energy.
2. Description of The Prior Art
In image forming apparatus, such as copying machines and laser-beam printers, a heat-roller type image fixing device is generally used for fixing a toner image to a toner sheet.
Such an image fixing device generally comprises a hollow fixing roller with a built-in heater serving as a heating source, and a pressure roller for pressing the fixing roller, to fix a toner image to a toner sheet by passing the sheet through a nip formed between the heating and pressure rollers.
However, in conventional heat-roller type image fixing devices, there is the disadvantage in that heat efficiency is low, since the fixing roller is heated in whole by radiation heat. There is also a design restriction in that it is necessary to arrange the heater at the center of the fixing roller in order to evenly heat the whole fixing roller.
In addition, when the fixing roller is heated in whole, the toner on the sheet which has passed through the nip remains fused by heat of the fixing roller around the nip. As a result, the toner fused by heat tends to be adhered to the fixing roller, so that there is a disadvantage in that so-called offset, in which the toner adhered to the fixing roller is again fixed to the sheet, tends to occur. It is also difficult for the sheet to be removed from the fixing roller due to adhesion of the toner remaining fused by the heat of the fixing roller around the nip, so that there is the disadvantage in that the sheet is wound onto the fixing roller causing jamming.
There is also the disadvantage in that electric power is uselessly demanded, since it is necessary to maintain the whole fixing roller at a predetermined temperature during stand-by time to prevent the offset.
There is also the disadvantage in that it takes a lot of warm-up time until the temperature of the fixing roller reaches a predetermined temperature after a main switch is turned on, so that there is a long waiting. If the temperature of the heating source is increased in order to decrease the warm-up time, there is the disadvantage in that the temperature within the image forming apparatus is also increased.
Furthermore, in some conventional image fixing devices, a temperature detector is secured to the heating source for detecting the temperature thereof. On the basis of the detected temperature, the temperature of the heating source is controlled to be constant, so that the temperature of the nip of the fixing roller is maintained within a temperature range suitable for the fixing. In such devices, since the heat produced by the heating source is transmitted to the inner wall of the fixing roller through an air layer, the temperature of the nip tends to differ from the temperature of the heating source detected by the detector. As a result, there is the disadvantage in that imperfect fixing and offset tends to occur, when the variation of temperature of the nip is so large that its temperature is out of the temperature range suitable for the fixing.